To expand on the other Anon's point
>Grey - Designed for original Game Boy, no specific colours in mind but if played on a GBC you can pick a colour palette to use
>Black - Designed with GBC in mind and some colours, but playable on an original Game Boy
>Clear (with bump) - GBC only, not compatible with older models
Pokemon is actually confusing here, from what I recall Red and Blue are equivalent to grey carts, Yellow is the equivalent to a black cart, Gold and Silver are also the equivalent to a black cart but oddly say GBC on the cover, implying it's exclusive, while Crystal actually is GBC exclusive.
